1. Field of the Invention
The present invention relates to an optical add/drop multiplexer, and more particularly to a bidirectional optical add/drop multiplexer and a wavelength division multiplexed ring network using the same.
2. Description of the Related Art
Recently, demands for Internet-based diverse multimedia services has increased rapidly, and research on an economic passive optical network (PON) that can provide a large capacity of information has been pursued actively. A passive optical network includes a central office (CO) for providing a service, a plurality of subscriber devices for receiving the service, and remote nodes (RNs), installed in areas adjacent to the subscribers, for connecting the central office with the subscriber devices. An outdoor network typically includes passive optical devices; however, it does not include the central office or subscriber devices. Since a network in which a single central office accommodates hundred of thousands to millions of subscribers distributed in a small region cannot be established, a metro-access network is typically used. In such network, a plurality of subscriber networks are constructed around remote nodes that can accommodate a specified number of subscribers, and a central office accommodates the nodes to communicate with them.
Recently, research efforts have been made regarding a ring structure that adopts a wavelength division multiplexed transmission technology, that secures the reliability of network, and that has an easy extensibility. Such research intends to accommodate the demand for increasing communication bandwidth in the metro-access network. In the metro-access network, the central office and the remote nodes are connected in a ring structure and communicate with one another using the optical signals having inherent wavelengths in the wavelength division multiplexed metro-access network. Since respective remote nodes communicate with a central office using such inherent wavelength optical signals, an add/drop function capable of (1) dropping and receiving the optical signal of the corresponding wavelength transmitted from the central office and (2) adding and transmitting the optical signal of the corresponding wavelength to be transmitted to the central office has been required.
FIG. 1 is a view illustrating the construction of a typical bidirectional optical add/drop multiplexer. Referring to FIG. 1, the bidirectional optical add/drop multiplexer 100 is arranged in transmission lines 170 and 175. The bidirectional optical add/drop multiplexer 100 includes first to sixth circulators 110 to 120 and first and second fiber Bragg gratings (FBGs) 130 and 135. The second and third circulators 112 and 114 and the first fiber Bragg grating 130 are arranged in a first optical path 140 that connects the first circulator 110 with the fourth circulator 116. The fifth and sixth circulators 118 and 120 and the second fiber Bragg grating 135 are arranged in a second optical path 145 that connects the first circulator 110 with the fourth circulator 116.
The first circulator 110 has first to third ports. The first port is connected to the sixth circulator 120, the second port is connected to the transmission line 170, and the third port is connected to the second circulator 112. The first circulator 110 outputs a first optical signal λ1 entering the second port to the third port and outputs a second optical signal λ2 entering the first port to the second port.
The second circulator 112 has first to third ports. The first port is connected to the third port of the first circulator 110, the second port is connected to the first fiber Bragg grating 130, and the third port is connected to a first drop terminal 160. The second circulator 112 outputs the first optical signal entering the first port to the second port and outputs the first optical signal entering the second port to the third port.
The first fiber Bragg grating 130 is arranged between the second port of the second circulator 112 and the second port of the third circulator 114 The first fiber Bragg grating 130 reflects the first optical signal. In other words, the first fiber Bragg grating 130 reflects the first optical signal from the second circulator 112 to the second circulator 112 and reflects the first optical signal from the third circulator 114 to the third circulator 114. The first and second fiber Bragg gratings 130 and 135 reflect optical signals of pre-selected wavelengths and transmit optical signals with wavelengths other than the pre-selected wavelengths.
The third circulator 114 has first to third ports. The first port is connected to a first add terminal 150, the second port is connected to the first fiber Bragg grating 130, and the third port is connected to a first port of the fourth circulator 116. The third circulator 114 outputs the first optical signal entering the first port to the second port and outputs the first optical signal entering the second port to the third port.
The fourth circulator 116 has first to third ports. The first port is connected to the third port of the third circulator 114, the second port is connected to the transmission line 175, and the third port is connected to the first port of the fifth circulator 118. The fourth circulator 116 outputs the first optical signal entering the first port to the second port and outputs the second optical signal entering the second port to the third port.
The fifth circulator 118 has first to third ports. The first port is connected to a third port of the fourth circulator 116, the second port is connected to the second fiber Bragg grating 135, and the third port is connected to a second drop terminal 165. The fifth circulator 118 outputs the second optical signal entering the first port to the second port and outputs the second optical signal entering the second port to the third port.
The second fiber Bragg grating 135 is arranged between the second port of the fifth circulator 118 and the second port of the sixth circulator 120 and reflects the inputted second optical signal. In other words, the second fiber Bragg grating 135 reflects the second optical signal from the fifth circulator 118 to the fifth circulator 118 and reflects the second optical signal from the sixth circulator 120 to the sixth circulator 120.
The sixth circulator 120 has first to third ports. The first port is connected to a second add terminal 155, the second port is connected to the second fiber Bragg grating 135, and the third port is connected to the first port of the first circulator 110. The sixth circulator 120 outputs the second optical signal entering the first port to the second port and outputs the second optical signal entering the second port to the third port.
The process of dropping the first optical signal from the transmission line 170 by the bidirectional optical add/drop multiplexer 100 will now be explained.
The first optical signal entering the bidirectional optical add/drop multiplexer 100 through the transmission line 170 passes through the first and second circulators 110 and 112, in order, and the signal is inputted to the first fiber Bragg grating 130. The first optical signal is then reflected by the first fiber Bragg grating 130 and exits to the first drop terminal 160 through the second circulator 112.
The process of adding the first optical signal to the transmission line 175 by the bidirectional optical add/drop multiplexer 100 will now be explained.
The first optical signal entering the first add terminal 150 is inputted to the first fiber Bragg grating 130 through the third circulator 114. The first optical signal is then reflected by the first fiber Bragg grating 130 and exits to the transmission line 175 through the third and fourth circulators 114 and 116.
The process of dropping the second optical signal from the transmission line 175 by the bidirectional optical add/drop multiplexer 100 will now be explained.
The second optical signal entering the bidirectional optical add/drop multiplexer 100 through the transmission line 175 passes through the fourth and fifth circulators 116 and 118, in order, and the second optical signal is inputted to the second fiber Bragg grating 135. The second optical signal is then reflected by the second fiber Bragg grating 135 and exits to the second drop terminal 165 through the fifth circulator 118.
The process of adding the second optical signal to the transmission line 170 by the bidirectional optical add/drop multiplexer 100 will now be explained.
The second optical signal entering the second add terminal 155 is inputted to the second fiber Bragg grating 135 through the sixth circulator 120. The second optical signal is then reflected by the second fiber Bragg grating 135 and exits to the transmission line 170 through the sixth and first circulators 120 and 110.
The bidirectional optical add/drop multiplexer 100, as described above, has the problems that it employs six expensive circulators 110 to 120, and the implementation cost, therefore, is high. In addition, the number of optical elements through which optical signals to be added or dropped pass is large, causing a great optical loss. Furthermore, crosstalk caused by optical signals having the same wavelength may occur due to the incomplete reflection by the fiber Bragg gratings 130 and 135. Specifically, if the optical signals to be dropped are not completely reflected by the fiber Bragg gratings 130 and 135, the transmitted optical signals cause crosstalk to the optical signals of the same wavelengths to be added after being reflected by the fiber Bragg gratings 130 and 135. Conversely, if the optical signals to be added are not completely reflected by the fiber Bragg gratings 130 and 135, the transmitted optical signals cause crosstalk to the optical signals of the same wavelengths to be dropped after being reflected by the fiber Bragg gratings 130 and 135. In such situation, crosstalk between optical signals of same wavelength can be avoided only by heightening the reflection rate of the fiber Bragg gratings 130 and 135 greatly.